The present invention generally relates to graphene-based polymer composites, and relates in particular to electrical conductivity enhancement of graphene-based polymer composites using a non-conductive filler.
Defect-free single layer graphene sheets consist of single atom thick sp2 bonded hexagonally arranged carbon atoms. These sheets display remarkable properties including exceptional in-plane electrical and thermal conductivity, high stiffness and tensile strength, optical transparency, negligible permeability to gases, and van der Waals transparency. The scientific and commercial interest in graphene is not restricted to the pristine monolayer, but includes related 2D materials that include few-layer graphene, multilayer graphene, graphene nanoplates, ultrathin 3D crystalline flakes with thickness <100 nm, and chemically modified forms such as graphene oxide. The essentially 2-dimensional nature of these materials along with their excellent properties makes them important as fillers, imparting useful functionalities into matrices. Polymers that display high conductivity have a variety of uses ranging from bulk applications such as anti-static mats and fuel lines, to specialty applications such as radiation shields, sensors and electrodes for batteries. While single layer graphene remains expensive and more suited for high end uses in electronic devices, opto-electronics, and supercapacitors, the electrical conductivity of the much lower cost graphene nanoplates (GNP)s is adequate for applications where a polymer must exhibit electrical conductivity. Graphene nanoplates have exceptional electrical, thermal, mechanical and barrier properties. Typical GNPs are several microns in lateral dimensions and consist of 8-10 layers of carbon atoms, providing aspect ratios (ratio of the lateral dimension to thickness) of the order of 103-104.
To achieve usable levels of electrical conductivity in an insulating material, a conducting filler must be loaded to a volume fraction beyond a percolation threshold. Graphene nanoplates are essentially two-dimensional structures. If allowed to rotate freely in a matrix, the ‘volume’ swept by it is that of a sphere of diameter corresponding to the lateral dimensions of the graphene nanoplates sheets, giving a theoretical volume loading at percolation that is well below that of spheres. If graphene nanoplates sheets are modeled as disks of aspect ratio (AR=disk diameter/thickness), the percolation threshold φc, under these conditions, is given byφc=1.5 φsphere/AR).  (1)
In Equation (1), φsphere is the percolation threshold for spheres, i.e., φsphere=0.29 φsphere=0.29 is for monodispersed spheres; that number is lower if there is polydispersity, but remains of the same order of magnitude). Since aspect ratio can take on values of the order of 104 for graphene nanoplates, the advantage of using these high aspect ratio conducting particles in lowering the volume loading at percolation becomes apparent compared to most common fillers that have aspect ratios close to 1. Providing such a low loading at percolation also has a significant benefit for mechanical properties, particularly under impact conditions, as filler materials can act as nucleation sites for crack growth, as well as lower material cost. GNP is now used as a filler material in polymer composites for various applications ranging from antistatic plastics, electrodes for batteries, electromagnetic interference (EMI) shields, field effect transistors (FET), solar cells, photovoltaics and various weight-sensitive aerospace and automotive applications.
While the volume loading at percolation is small for sheet like materials, van der Waals attraction between these sheets cause rapid agglomeration and dispersing these sheets in a polymer remains a major challenge. Also, interfacial incompatibility between graphene and polymers results in a large drop in the flexural strength and toughness of the composite compared to the native polymer.
There remains a need, therefore, for a resilient graphene-based polymer composite having low graphene loading while providing high electrical conductivity.